Specular neutron reflection and surface tension have been used to investigate the composition and structure of the surfactant-polymer mixture of sodium dodecyl sulfate, SDS, hexaethylene glycol monododecyl ether, C12E6, and the cationic polymer poly(dimethyldiallylammonium chloride), dmdaac, at the air-water interface. The variation of surface tension with surfactant concentration shows a similar complex behavior to that observed and previously reported for SDS/poly-dmdaac. There is a marked increase in the surface tension between the concentrations normally associated with the critical aggregation concentration (cac) and the critical micellar concentration (cmc). The extent of the increase depends strongly upon the solution composition and is most pronounced for solutions rich in SDS. The neutron reflectivity measurements show that this marked increase in surface tension is strongly associated with surfactant composition at the surface and with the amount of polymer at the interface. In this region of surface tension increase, between nominally the cac and cmc, the amount of SDS and polymer at the surface decreases markedly. This is attributed to the competition between the formation of surface and solution surfactantpolymer complexes that are rich in SDS.